Year 9 Science · Unit 3 · Lesson 2
Challenge Worksheet
Learning Goals
1. Define a closed system and an open system. Which kind is a real roller coaster, and why?
Scenario: a roller coaster ride
A roller coaster cart is pulled to the top of the first hill and let go. It rolls down to the bottom and up a second, lower hill. Table 1 shows the cart’s energy at four points. Gravitational potential energy (GPE) is measured from the lowest point of the track.
| Point | Height (m) | GPE (kJ) | Kinetic energy (kJ) | GPE + kinetic energy (kJ) |
|---|---|---|---|---|
| A, top of the first hill | 30 | 180 | 0 | 180 |
| B, halfway down | 15 | 90 | 82 | |
| C, bottom of the track | 0 | 0 | 165 | |
| D, top of the second hill | 25 | 150 | 10 |
Source: sample data calculated for this worksheet. The cart’s GPE is 6 kJ for every metre of height.
2. Complete the three missing values in Table 1.
B total kJ
C kinetic energy kJ
D total kJ
3. The total in the last column falls from A to D. Explain why, and state how much energy has been transferred to the surroundings between A and D.
4. The designers want to add a third hill, 29 m high, straight after point D. Use Table 1 to decide whether the cart can reach the top of this hill without a motor. Show your working.
Energy needed at the top kJ
Energy the cart has at D kJ
Find the mistakes: a student’s explanation
“At point A the cart has no energy, because it is not moving. As it rolls down, gravity creates kinetic energy. At the bottom all the energy is kinetic, so the total energy is biggest there. The energy that goes missing along the track is destroyed by friction.”
5. Identify four errors in the student’s explanation. Correct each one, using Table 1 where you can.
| Error | What the student wrote | Correct physics |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 |
Someone claims…
An inventor advertises a sealed device that “makes more electricity than it uses”. For each test, the device starts and ends in the same state, and testers account for every energy transfer into it. Table 2 compares that total input with the electrical output.
| Test | Total energy in, all transfers (kJ) | Electrical energy out (kJ) |
|---|---|---|
| 1 | 50 | 62 |
| 2 | 50 | 61 |
| 3 | 50 | 63 |
Source: sample data invented for this worksheet.
6. Calculate the average electrical energy out, and how much the output appears to exceed the total energy input in each test.
Average energy out kJ
Apparent extra energy kJ
7. Evaluate the inventor’s claim. Use the law of conservation of energy, suggest two things that could explain the results in Table 2, and describe how you would test the machine fairly.